You dont have to mill the timing cover unless you deck the block. Pull the valve cover and see if your timing chain is loose. You can also look in with a flash light and see if the guide is broken. If the guide is broken then there is a good chance that your cover is junk. If the guide is intact, It is doubtful that you cover is damaged. This would be the straight guide. Dniel is correct though. You should be able to remove the cover with the head still on. Be sure to change all timing parts and the front seal if you go through the trouble.
